The present invention relates to the assembly of a principal semiconductor component with an auxiliary circuit. It applies more particularly to power semiconductor components intended for switching.
The method of assembly in accordance with the present invention is more specifically relative to the association of a circuit of the destorage diode type with integrated amplification transistor circuits of the Darlington type or with thyristor circuits of the gate amplifying type. Thus, the nature and the structure of these special circuits will be recalled below with reference to FIGS. 1 to 6.
FIG. 1 shows a Darlington-type transistor circuit. This circuit comprises a first transistor or power transistor T.sub.1 associated with a control transistor or pilot transistor T.sub.2. The collectors of transistors T.sub.1 and T.sub.2 are interconnected and the emitter of transistor T.sub.2 is connected to the base of transistor T.sub.1. This circuit comprises three terminals: principal terminals A and B to which the signal to be switched is applied and a control terminal C. Terminal A corresponds to the collector of transistors T.sub.1 and T.sub.2, terminal B to the emitter of transistor T.sub.1 and terminal C to the base of transistor T.sub.2. For the user, this Darlington circuit appears as a particularly sensitive transistor.
FIG. 2 shows an amplifying gate thyristor circuit. This circuit comprises a main thyristor or power thyristor Th.sub.1 and an auxiliary thyristor Th.sub.2. The anodes of these thyristors are interconnected and the cathode of thyristor Th.sub.2 is connected to the gate of thyristor Th.sub.1. This circuit comprises three terminals: two principal terminals A and B to which the signal to be switched is applied and a control terminal C. Terminal A corresponds to the anodes of thyristors Th.sub.1 and Th.sub.2, terminal B corresponds to the cathode of thyristor Th.sub.1 and terminal C corresponds to the gate of thyristor Th.sub.2. This circuit behaves for a user like a single thyristor which is particularly sensitive and rugged on closing.
Thus a great similarity is seen between the Darlington type transistor circuits and the amplifying gate thyristor circuits. In both cases, the amplification stage formed by transistor T.sub.2 or thyristor Th.sub.2, respectively, could be a multiple stage comprising several transistors or several thyristors. In FIGS. 1 and 2, the control terminal of the main component is moreover shown by the same reference B. The similarity between the circuits of FIGS. 1 and 2 goes even further if we consider their construction in the conventional integrated form which is illustrated in FIGS. 3 and 4.
FIG. 3 shows a Darlington circuit transistor structure. The main transistor or power transistor T.sub.1 is shown only partially and comprises an N collector layer 1, a P base layer 2 and an N emitter layer 3. The emitter is coated with an emitter metalization designated by the reference B and the collector layer is coated with a collector metalization designated by the reference A, with interpositioning of an overdoped N.sup.+ layer 4. The amplifying structure is formed by adding an N layer 5 (annular like layer 3 in the example shown) which corresponds to the emitter zone of the pilot transistor T.sub.2. A central metalization C corresponds to the control terminal illustrated in FIG. 1 and a metalization D connecting the emitter layer of the pilot transistor to the base layer of the main transistor corresponds to terminal D of FIG. 1. Layer 6 which appears between the metalizations of the main face represents schematically a layer of a passivation material.
FIG. 4 shows very schematically an amplifying gate thyristor structure using the circuit of FIG. 2. This figure will not be described in detail. We will simply note its great similarity with FIG. 3. The essential difference resides in the fact that N.sup.+ layer 4 of FIG. 3 is replaced by a P layer 7. It will however be noted that the term thyristor should be understood as covering any four layer NPNP structure, i.e. not only conventional structures such as shown in FIG. 4 but also those inherent in amplifying gate thyristors. The thyristors may also have asymmetric NPNN.sup.+ P type structures and structures using different forms of interdigitation not only for the power stage but also for the amplifier stages. Four-layer structures may also be bidirectional (Triac type).
The assembly of power structures such as those shown in FIGS. 3 and 4 often uses a technology of the pressed-contact type on the silicon disk, this latter being possibly previously alloyed or not on a counter-electrode made from molybdenum or tungsten for example. For putting the contacts under pressure, two approaches are currently used. One uses a so-called "screw" case integrating the pressurizing system by means of internal springs (so called conical washers). The other uses slightly deformable flat case technique, the pressure on the component then being applied directly by the user during insertion of the component into a convector (these cases are currently called double-face pressed cases). In each case, it is also possible to provide the electrical connection of the control by putting the control electrode contact (C) under pressure with a counter-electrode electrically accessible by the user. Whatever the type of case and component, the qualitative configuration of the contact tappings on the upper face is identical.
After these reminders about the nature and the structure of the particular power circuits formed by the Darlington assemblies of amplifying gate thyristors, an example will now be given of an auxiliary component which users often desire to use in relation with these particular power components. The structures, such as shown in FIGS. 1 and 2, do not lead to an optimum electrical configuration for the overall dynamic behavior of the integrated structure and more specifically during electrical opening. The origins of this limitation are, as is well known, essentially associated with the fact that it is difficult to assist electrically the opening of the component from the control electrode C accessible to the user of a conventional amplifying structure. In fact, in the case of the polarities of layers shown in FIGS. 3 and 4, it is desirable for facilitating the opening of the component to be able to apply a negative potential to the metalization D of the power stage so as to, for example, electrically extract by means of this electrode a part of the charges induced in the power stage during the preceding conduction. In fact, in the case of an integrated structure such as shown in FIGS. 3 and 4, the junction between layers 5 and 2 of the pilot amplifier stage is rapidly inhibited and opposes the extraction of a negative current on the control electrode of the power stage of the integrated structure, which considerably delays the dynamic opening possibilities of this latter.
To resolve this difficulty, it is known, as shown schematically in FIGS. 5 and 6, that it is desirable to provide a shunt diode also called recovery diode or destorage diode between terminals D and C previously defined.
In the prior art, for assembling the auxiliary circuit formed by the destorage diode with the principal component, formed from a Darlington-type transistor or from an amplifying gate thyristor, three approaches have been used.
The first approach consists in integrating the destorage diode in the silicon disk of the principal component. This first approach seems the most attractive more particularly in the case of low-power devices for which the cost of construction constitutes one of the main industrial objectives. This approach presents nevertheless numerous electrical disadvantages because, on the one hand, of the presence of parasite coupling elements inherent in the integration and, on the other hand, of the absence of flexibility for adjusting the electrical characteristics of the destorage diode to the needs of the user. This limitation is particularly felt in the case of high-power devices.
The second approach consists in making terminals C and D accessible on the outside of the case of the power component and in wiring the destorage diode externally of the case. This second approach requires the provision of new specific cases, which constitutes a not inconsiderable industrial difficulty for the component manufacturer and imposes on the other hand on the user additional electric wiring operations.
The third approach consists in making terminal D accessible inside the case of the power component and in inserting the destorage diode in the case and in wiring it therein internally. This third approach leads to delicate technological wiring operations inside a case and requires, on the other hand, the use of overdimensioned cases for housing the destorage diode. There thus exists an insertion area lost for the power stage, i.e. for the performance of the component. In fact, the case forms one of the not inconsiderable cost elements for a power component.